Alachlor dechlorination prior to an electro-Fenton process: Influence on the biodegradability of the treated solution

This study investigates alachlor herbicide removal by electro-Fenton (EF) process with and without a previous dechlorination step by electro-reduction (ER-EF). The objective is to evaluate the relevance of dechlorination before electro-Fenton according to the biodegradability evolution and the energy consumption. The influence of the applied current, the ferrous ions concentration and the air flow rate on the production of hydrogen peroxide and on the general behavior of alachlor removal by EF oxidation was studied and optimized EF conditions were chosen. Scavenger tests were performed to determine the contribution of hydroxyl radicals ((OH)-O-center dot), superoxide radicals (O-center dot(2)-) and sulfate radicals ((SO4-)-S-center dot) during alachlor degradation. Some intermediate products formed during degradation by EF of alachlor and of the electroreduced solution were identified and the evolution of small organic acids was examined. Possible pathways of alachlor degradation and of deschloroalachlor, the mean byproduct from ER, were proposed on the basis of the EF reaction. EF treatment significantly improved the biodegradability of electrolyzed solutions with BOD5/COD ratio increasing from 0 to 0.4 after 0.5 h oxidation and can be further enhanced by the application of ER prior to EF, leading to a maximum value for the BOD5/COD ratio of 0.7 that can be due to the absence of chloroacetic acid found during the EF treatment. The energy consumption of the different electrochemical processes was also evaluated.